This invention relates generally to frequency spectrum analyzers and more particularly to analyzers of such type which are useful in real time signal processing.
As is known in the art, real time signal processors have been used extensively in many applications such as in sonar and radar. For example, a real time signal processor may be used in a radar receiver for determining the Doppler frequency associated with a detected target. In such a radar receiver a bipolar video signal is produced in response to each one of a train of transmitted pulses. Such pulses are transmitted at a predetermined pulse repetition frequency, or "PRF". A predetermined time after each one of the pulses in the train thereof is transmitted, the bipolar video signal is sampled and stored to obtain a set of samples of such signal, each sample in the set thereof corresponding to a return within a particular one of a number of range cells. The rate of change in amplitude of the samples within each one of the range cells is indicative of the Doppler frequency of any object in each one of the range cells. As is known, such Doppler frequency may be obtained by passing each set of samples through a frequency spectrum analyzer. The described real time signal processing has been implemented using various analog and digital processing apparatus. However, such implementations generally require relatively complex and costly components.
As is described in articles entitled "High-Speed Spectrum Analyser Using a Pulse Compression Technique" by J. A. Edwards and M. J. Withers, published in the Proceedings of the IEE, Vol. 114, No. 11, November 1967 and "The Design and Application of Highly Dispersive Acoustic-Surface Wave Filters" by H. M. Girard, W. R. Smith, W. R. Jones and J. B. Harrington published in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-21, No. 4, April 1973, the frequency of an input signal may be determined by mixing such input signal with a linear frequency modulated (FM) signal, i.e. a "chirp" pulse, then passing the resulting signal through a pulse compression filter, and finally measuring the time of occurrence of the compressed pulse produced by such pulse compression filter relative to a reference time, t.sub.O. Thus, if the frequency of the input signal is zero, the compressed pulse occurs at the reference time t.sub.O, but if the frequency of the input signal is f.sub.d, the compressed pulse occurs at a time f.sub.d /S after the reference time t.sub.O, where S is the ratio of the change in the frequency of the chirp pulse (i.e. dispersive bandwidth) to the chirp pulse time duration (i.e. the dispersive time).
In such a system the input signal, the frequency spectrum of which is to be analyzed, is swept over a range of frequencies 2.DELTA.f (chirp pulse dispersive bandwidth) in a time duration 2.DELTA.T (the chirp pulse dispersive time) where .DELTA.f is the frequency bandwidth of such input signal. The pulse compression filter has a compressive bandwidth also equal to .DELTA.f and a compressive time equal to .DELTA.T. Because the input signal is swept over a bandwidth greater than the compressive bandwidth of the pulse compression filter, here 2:1 greater, a 3 db loss in power in the compressed pulse will result, thereby reducing the signal to noise (S/N) ratio of the received signal if such technique were used in a radar system. In order for the chirp pulse and the input signal to be properly mixed, the chirp pulse must occur during the time of the input signal. Further, in order to properly pulse compress the mixed signals, the ratio of the dispersive bandwidth to the dispersive time (i.e. the ratio S) must be equal in magnitude to the ratio of the compressive bandwidth to the compressive time of the pulse compression filter. Therefore, if such a system were to be used in a pulse Doppler radar, where the input signal is the bipolar video signal comprised of a number of radar returns (say 64 radar returns) taken at the radar PRF rate, say (1/200) MHz, a dispersive time of several milliseconds would be required for the chirp pulse. As is known in the art one convenient pulse compression filter includes a surface acoustic wave (SAW) delay line. In order to pulse compress a signal having several milliseconds time duration such SAW delay line would require a length in the order of several feet thereby making use of such SAW delay line impractical in a pulse Doppler radar.